Conventional photographic materials based on silver halide are used for a large variety of applications. For instance, in the pre-press sector of graphic arts rather sensitive camera materials are used for obtaining screened images. Scan films are used for producing colour separations from multicolour originals. Phototype setting materials record the information fed to phototype- and image setters. Relative insensitive photographic materials serve as duplicating materials usually in a contact exposure process. Other fields include materials for medical recording, duplicating and hard copy, X-ray materials for non-destructive testing, black-and-white and colour materials for amateur- and professional still photography and materials for cinematographic recording and printing.
Silver halide materials have the advantage of high potential intrinsic sensitivity and excellent image quality. On the other hand they show the drawback of requiring several wet processing steps employing chemical ingredients which are suspect from an ecological point of view. As a consequence it is undesirable that depleted processing solutions would be discharged into the public sewerage; they have to be collected and destroyed by combustion, a cumbersome and expensive process.
In the past several proposals have been made for obtaining an image that can be formed using only dry development steps without the need of processing liquids as it is the case with silver halide photographic materials.
A dry imaging system known since quite a while is 3M's dry silver technology. It is a catalytic process which couples the light-capturing capability of silver halide to the image-forming capability of organic silver salts.
As a further alternative for conventional silver halide chemistry dry imaging elements are known that can be image-wise exposed using an image-wise distribution of heat. When this heat pattern is indirectly generated by the conversion of radiation, e.g. laser radiation, into heat these types of dry imaging elements are called heat mode materials. When the heat pattern is provided directly, e.g. by means of a thermal head, these elements are called thermal recording materials or thermographic materials. Both types of elements offer the advantage in addition to an ecological advantage that they do not need to be handled in a dark room nor any other protection from ambient light is needed. Heat mode recording materials, based on change of adhesion, are disclosed in e.g. U.S. Pat. No. 4,123,309, U.S. Pat. No. 4,123,578, U.S. Pat. No. 4,157,412, U.S. Pat. No. 4,547,456 and PCT applications WO 88/04237 and WO 93/03928.
In still another type of heat mode recording materials information is recorded by creating differences in reflection and/or transmission in the recording layer. The recording layer has high optical density. The conversion of radiation into heat brings about a local temperature rise, causing a change such as evaporation or ablation to take place in the recording layer. As a result, the irradiated parts of the recording layer are totally or partially removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346). In a preferred embodiment the recording layer of such heat mode recording materials is constituted by a thin metal, e.g. a bismuth layer.
Still another type of non-conventional materials as alternative for silver halide is based on photopolymerisation. The use of photopolymerizable compositions for the production of images by information-wise exposure thereof to actinic radiation is known since quite a while. All these methods are based on the principle of introducing a differentiation in properties between the exposed and non-exposed parts of the photopolymerizable composition e.g. a difference in solubility, adhesion, conductivity, refractive index, tackiness, permeability, diffusibility of incorporated substances e.g. dyes etc. Some of the thus produced differences may be subsequently employed in a dry treatment step to produce a visible image and/or master for printing e.g. a lithographic or electrostatic printing master. in U.S. Pat. No. 4,751,167 an image forming material, working according to photo mode, is described comprising a substrate, an image producing layer constituted by a metal or metal compound layer, and a photopolymerisable layer containing a specific type of monomer. When not yet exposed the adhesion between the image producing layer and the photosensitive layer is lower than the adhesion between the substrate and the photosensitive layer, while upon exposure this adhesion balance is reversed. As a result, after peeling apart by means of a cover film a positive metal image is removed with the photosensitive layer and the cover sheet in the exposed areas while a negative metal image is retained on the substrate in the unexposed areas. A disadvantage of this method is the fact that both formed metal images are unprotected to physical damages, e.g. to scratches.
The present invention further extends the teachings on the combination of photopolymerisation and metal image forming layer.
It is an object of the present invention to provide an unconventional image forming material, based on photo mode, which constitutes an alternative to conventional image-setting films and contact duplicating films based on silver halide.
It is a further object to provide a process for the formation of a positive metal image which is protected against physical damages.